This invention relates to a compressor system and more particularly, to a compressor control system and method of operating a compressor system comprising multiple throttled inlet rotary screw compressors.
Compressed fluids, such as air, are commonly used in an industrial environment and serve as a power source for various machines and tools. The actual demand for such compressed fluids typically fluctuates throughout the course of a standard work day due to the varying load requirements of each machine and tool. In order to fulfill the demand for a wide variation in load level, the system for supplying the compressed fluid typically includes a plurality of various sized compressors. The compressors are usually connected in parallel and/or series in order to produce a total capacity that can adequately satisfy the anticipated demand.
The compressors are typically started in sequence beginning with the smallest sized compressor and ending with the largest sized compressor. Thereafter, the compressors are cycled xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d (i.e., loaded or unloaded) in response to the pressure demands required by the load(s) within the system. For example, in an industrial environment, the activation of one or more pneumatically powered tools connected to a compressed air system results in an outflow of compressed air, thereby reducing the overall system pressure. In order to maintain the desired system pressure, it may be necessary to load another compressor onto the system. Similarly, if a machine requiring compressed air is turned xe2x80x9coffxe2x80x9d, the system will have an over abundance of pressurized fluid, thereby increasing the system pressure. Hence, it may be necessary to unload one or more compressors. The issue, therefore, becomes which compressor should be loaded or unloaded (i.e., added or removed from the compressor system).
A pressure responsive control system typically includes a controller that is wired to measure the system pressure. When the system pressure drops below or climbs above a predetermined pressure set-point, the controller loads or unloads the next available compressor. This process is repeated as often as necessary, or until all of the compressors are either loaded or unloaded, in order to compensate for the change in system demand. Similarly, rather than controlling the compressor system in response to the system pressure, the compressor system could be responsive to a flow sensor that measures the fluid flow.
Whether responding to the system pressure or fluid flow within the system, it may be advantageous to initially sequence the loading and unloading of the compressors in order to produce an electrically efficient compressor system. Particularly, the operator of the system may recognize that the load requirements increase as the day progresses. Therefore, certain compressors are loaded and/or unloaded accordingly such that the pressure or flow capacity of the system increases to satisfy the anticipated load requirements. In other words, the sequence is typically based upon an estimate of the anticipated system demand such that the estimate is close as possible to the actual system requirements. The goal of estimating the actual system demand and loading the appropriately sized compressors is to provide a moderately efficient system by minimizing the unused capacity of the system, thereby minimizing the wasteful use of electrical power.
After initially loading the compressors according to the sequence, the compressors are loaded and/or unloaded in response to the pressure or fluid flow sensors to more particularly satisfy the demands of the system. For example, if after the initial sequence of compressors is loaded and the system senses that it requires additional pressure, an additional compressor may be loaded to the system to satisfy the immediate demand. Likewise, if the system senses that it has an over abundant pressure capacity, a compressor may be unloaded to reduce the overall system pressure.
However, controlling the compressor system solely in response to the pressure differential between the actual system pressure and the pressure requirement of the system may not be as electrically efficient as originally believed. Specifically, although the system may produce fluid with sufficient pressure, the volume of fluid being produced may be inadequate or substantially excessive in comparison to what is actually required. Similarly, controlling the compressor system solely in response to a change in the fluid flow rate may not be the most electrically efficient control method because although the system may produce the required volume of fluid, the pressure of fluid being produced may be substantially higher or lower than what is actually required.
Additionally, compressors may require a significant amount of time to produce fluid at their rated pressure capacity and flow rate. Moreover, during such period, the compressors utilize a significant amount of electrical energy, which translates into a high operating cost. Furthermore, upon the compressors attaining their rated capacity, the present control systems do not insure that the appropriate blend of compressors is loaded to the system in order to produce the most suitable pressure and volumetric flow rate capacity. In other words, controlling the compressors in response solely to a change in pressure or fluid flow may produce excessive amounts thereof. These unnecessary quantities of fluid flow and pressure are the by products of the compressors utilizing an over abundance electrical energy. Thus, controlling the compressors in response solely to a change in pressure or fluid flow utilizes an unwarranted amount of electrical power, thereby producing an inefficient compressor control system.
It is an object of the invention to produce a more efficient compressor control system.
It is an other object of the invention to produce a compressor control system that does not merely control the operation of compressors as a function of the system pressure.
It is an other object of the invention to produce a compressor control system that does not merely control the operation of the compressors as a function of the system""s volumetric flow rate capacity.
It is a further object of the invention to produce a compressor control system comprising of a plurality of throttled inlet rotary screw compressors.
It is a further object of the invention to produce a compressor control system comprising a plurality of throttled inlet rotary screw compressors, wherein the control system does not control the operation of the compressors by sensing only the compressor systems pressure or its volumetric flow rate.
It is even a further object of the invention to produce a control system for a plurality of throttled inlet rotary screw compressors, wherein the control system does not utilize a flow meter to determine the compressor system""s volumetric flow rate.
The present invention is a more efficient compressor control system and a more efficient method of operating a multiple compressor system because the method and control system are a function of both the system pressure and the volumetric flow rate of the system. Thus, a compressor is loaded or unloaded from the compressor system after sensing both the actual pressure and volumetric flow rate of air within the compressor system. The actual flow rate of the compressor system is compared to the online flow rate capacity, and the set-point pressure is compared to the actual pressure of the compressor system. Upon completing these two comparisons, a determination is made as to which, if any, compressors should be loaded or unloaded. These two comparisons allow the control system to load or unload the compressor that will produce the most efficient compressor system. If the pressure of the compressor system is greater than the pressure set-point and the flow rate capacity for one of the compressors is less than the excess flow rate for the compressor system, that particular compressor will be unloaded from the compressor system. Additionally, if the actual pressure of the compressor system is less than the set-point pressure and the actual flow rate of the compressor system is equal to or greater than the online flow rate capacity of the compressor system, then a compressor will be loaded to the compressor system.
The control system of the present invention does not use a typical flow meter to measure the system""s actual flow rate. Rather, the control system of the present invention senses the inlet pressure of each loaded throttled inlet rotary screw compressor and converts that inlet pressure to an outlet volumetric flow rate. Assuming is the compressor system includes multiple throttled inlet rotary screw compressors, the controller adds all of the converted volumetric flow rates to estimate the system""s actual flow rate. Because the control system of the present invention can calculate the system""s actual volumetric flow rate by sensing the inlet pressure of each operating compressor, the control system does not require a discrete flow meter.
Accordingly, the present invention relates to a method of operating a compressor system having a plurality of throttled inlet rotary screw compressors, the method comprising the steps of establishing a set-point pressure, measuring the actual pressure of the fluid in the compressor system, determining which of the throttled inlet rotary screw compressors are operating, calculating the actual volumetric flow rate of fluid in the compressor system by: sensing the inlet pressure of each operating throttled inlet rotary screw compressor, converting the inlet pressures to corresponding outlet volumetric flow rates and summing the outlet volumetric flow rates, determining the online volumetric flow rate capacity, wherein the online volumetric flow rate capacity is equal to the sum of the corresponding rated volumetric flow rate capacities for each of the operating throttled inlet rotary screw compressors, determining the excess volumetric flow rate of the compressor system, wherein the excess volumetric flow rate is equal to the online volumetric flow rate capacity minus the compressor system""s actual volumetric flow rate, and unloading one of the plurality of rotary screw compressors upon sensing that the actual pressure of the fluid in the compressor system is greater than the set-point pressure and the corresponding rated volumetric flow rate capacity for the one throttled inlet rotary screw compressor is less than the excess volumetric flow rate.
The present invention also relates to another method of operating a compressor system having a plurality of throttled inlet rotary screw compressors, the method comprising the steps of establishing a set-point pressure, measuring the actual pressure of the fluid in the compressor system, determining which of the rotary screw compressors are operating, calculating the actual volumetric flow rate of fluid in the compressor system by: sensing the inlet pressure of each operating throttled inlet rotary screw compressor, converting the inlet pressures to corresponding outlet volumetric flow rates; and summing the outlet volumetric flow rates, determining the online volumetric flow rate capacity, wherein the online volumetric flow rate capacity is equal to the sum of the corresponding rated volumetric flow rate capacities for each of the operating throttled inlet rotary screw compressors, loading one of the plurality of throttled inlet rotary screw compressors upon sensing that the actual pressure of the fluid in the compressor system is less than the set-point pressure and that the actual volumetric flow rate is equal to or greater than the online volumetric flow rate capacity.
The present invention also relates to a control system for controlling a compressor system, comprising a plurality of throttled inlet rotary screw compressors each having an inlet and outlet and being capable of compressing a fluid at a rated volumetric flow rate and pressure, each of the throttled inlet rotary screw compressors emitting an operational signal indicative of whether the corresponding throttled inlet rotary screw compressor is operating, a plurality of pressure sensors, wherein one of the pressure sensors is located upstream of each of the throttled inlet rotary screw compressors, another pressure sensor for sensing the actual pressure of the fluid in the compressor system, a controller having a programmed set-point pressure, the controller having programmed values representing each of the throttled inlet rotary screw compressors"" rated volumetric flow rate capacities and rated pressures, the controller receiving the operational signals from the plurality of throttled inlet rotary screw compressors, the controller receiving pressure signals from the pressure sensors located upstream of the inlets and calculating the actual volumetric flow rate of fluid in the compressor system from the pressure signals, the controller determining an online volumetric flow rate capacity, wherein the online volumetric flow rate capacity is equal to the sum of the rated volumetric flow rate capacities for each of the operating throttled inlet rotary screw compressors, the controller determining an excess volumetric flow rate for the compressor system, wherein the excess volumetric flow rate is equal to the online volumetric flow rate capacity minus the actual volumetric flow rate, the controller producing an unloading signal for one of the plurality of compressors upon sensing that the actual pressure of the fluid in the compressor system is greater than the set-point pressure and the corresponding rated volumetric flow rate capacity for the one compressor is less than the excess volumetric flow rate.
The present invention also relates to another control system for controlling a compressor system, comprising a plurality of throttled inlet rotary screw compressors each is having an inlet and an outlet and being capable of compressing a fluid at a rated volumetric flow rate and pressure, each of the compressors emitting an operational signal indicative of whether the corresponding throttled inlet rotary screw compressor is operating, a plurality of pressure sensors, wherein one of the pressure sensors is located upstream of each of the throttled inlet rotary screw compressors, another pressure sensor for measuring the actual pressure of the fluid in the compressor system, a controller having a programmed set-point pressure, the controller having programmed values representing each of the throttled inlet rotary screw compressors"" rated volumetric flow rate capacities and rated pressures, the controller receiving the operational signals from the plurality of throttled inlet rotary screw compressors, the controller receiving pressure signals from the pressure sensors located upstream of the throttled inlet rotary screw compressors and calculating the actual volumetric flow rate of fluid in the compressor system from the pressure signals, the controller determining an online volumetric flow rate capacity, wherein the online volumetric flow rate capacity is equal to the sum of the rated volumetric flow rate capacities for each of the operating throttled inlet rotary screw compressors, the controller producing a loading signal for one of the throttled inlet rotary screw compressors upon sensing that the actual pressure of the fluid in the compressor system is less than the set-point pressure and the compressor system""s actual volumetric flow rate is equal to or greater than the online volumetric flow rate capacity.
The foregoing features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof as illustrated in the accompanying drawings.